Baseball in April many times determines the outlook for your favorite club's season. A good April will help carry momentum into the future months while giving you an advantage of having more breathing room in the standings. If you get off to a bad start in April, and say you're 7 games below .500, it becomes much more difficult to rebound and have a successful season. Not to mention April can have a major effect on your position during the trade deadline.
The beginning months of baseball still matter. There are some fans that count down until pitchers and catchers report. Plus, sometimes the first few games can be indicative of the team's season. Fans know that every game is important to the overall season, even the ones in the very beginning. Ticket sales may not be as high, but there are still people going to the games.
Once the regular season starts, every game matters. The games that are won or lost in April help make or break a team in September. True that it may take a few weeks or even months for a player to warm up or get hot, but I think every fan loves opening day just as much as a division deciding game in September.
There is such a thing as setting the tone in a competition, and baseball is no different. If one team can sit up and make other teams sit up and take notice, then that team has the momentum. The key is not to get too caught up in that one month though. Everyone is fresh, uninjured and likely to play better for that reason.
April does not matter in the MLB for the fans. Nobody really starts paying attention to baseball until about two or three months into the season. The season is too long and nobody wants to watch all those games. August through October are the best parts of the MLB season.